Elevator dispatching software causes a particular elevator car in a bank of elevators to be sent to a floor in response to a user pressing a hall call button at that floor. Traditionally, a hall lantern will illuminate just prior to the car doors opening in order to inform the user as to which car will service his call. The dispatching software chooses which car to assign to the call according to a variety of elevator system parameters, such as average waiting time, maximum waiting time, average travel time, etc. It is possible for the values of these system parameters to change between the time the call is registered and the time the call is serviced. Therefore the software may reassign the call to other cars many times before the call is serviced. The user does not notice the reassignment because the hall lantern is lit only just before the car arrives.
Unlike traditional car assignment techniques, instantaneous car assignment (ICA) informs a user instantaneously (or shortly thereafter) as to which car will service his hall call. The ICA assignment technique is more demanding in that it is not acceptable to repeatedly reassign different cars to a hall call since this will cause continuous lighting and unlighting of hall lanterns causing confusion for the hall passengers. Accordingly, it is important to provide an initial assignment that is as good as possible. This includes determining the effect of a hall call on other registered calls for the elevator car.